1. Field of Invention
The invention relates to a transforming device of a power source and a transformer thereof.
2. Related Art
Currently, electronic circuits are widely applied in electronic devices including transforming devices of power sources, such as power supplies and electric energy converters. However, this kind of circuit often operates via high-frequency switching so that electromagnetic interference (EMI) tends to occur and thus influences the operation of the electronic device. EMI may be divided into radiated EMI and conducted EMI according to the mode of interference transfer. Radiated EMI is transferred directly in an open space, and the conducted EMI is transferred through wires.
Conducted EMI may further be divided into common-mode noise EMI occurring when all wires have the same current flow direction, and differential-mode noise EMI occurred when two wires have opposite current flow directions according to the noise current conducting path.
In order to effectively eliminate the EMI, an inductor for eliminating the noises is usually disposed in the electronic device according to the kinds of the noise. For example, if the common-mode noise has to be eliminated, a common mode inductor and a capacitor are disposed in the electronic device so that the common-mode noise may be eliminated. If a differential-mode noise has to be eliminated simultaneously, a differential-mode inductor and another capacitor are needed into the electronic device so that the differential-mode noise may be eliminated. However, with the development of the electronic technology, the multi-functionality and required power density requirements placed on electronic devices are steadily increased. Correspondingly, the size of the inductor needs to be minimized. The most effective method for reducing the size and the cost of the inductor is to reduce the original noise of the electronic device. Because the common mode inductor available on the market is large and complicated to manufacture, decreasing the common-mode noise has become an important subject of enhancing the transforming device.
FIG. 1 shows an equivalent circuit of a conventional transforming device 1. Referring to FIG. 1, the transforming device 1 includes a transformer 11, a bridge rectifier 12, a capacitor 13 and a transistor 14, which are electrically connected to one another. The transformer 11 has a primary winding 111 wound outside a magnetic element (not shown), and a secondary winding 112 wound outside the primary winding 111. The transforming device 1 receives an AC voltage V1, which is externally inputted to the bridge rectifier 12 and inputted to the transistor 14 through a rectified voltage. The transistor 14 performs switching operations according to an externally inputted pulse width modulation (PWM) signal P1. The rectified voltage is subsequently transferred to the transformer 11 to generate a DC voltage V2. The common-mode noise is usually caused by the voltage trip of the secondary winding 112 or the primary winding 111 of the transformer 11, or caused by the coupling of the parasitic parameters.
FIG. 2 shows the measured results of the common-mode noise obtained from the transforming device 1. At present, the frequency range of the standard EMI is between 0.15 MHz and 30 MHz. As shown in the drawing, it is obtained that the common-mode noise value of the transforming device 1 is about 76 dB.
The prior arts mainly use the following two methods to eliminate the common-mode noise. As shown in FIG. 3, the first method for eliminating the common-mode noise is to add a winding 113 between the primary winding 111 and the secondary winding 112 in the transformer 11, and the winding 113 is electrically connected to a capacitor 15. Consequently, a phase complementary voltage is generated between the primary winding 111 and the secondary winding 112 and a common mode current for offsetting the common mode current of the transformer 11 is generated according to the phase complementary voltage. However, this method complicates the design of the transformer 11 due to the added winding 113, and also increases the manufacturing cost.
As shown in FIG. 4, the second method for eliminating the common-mode noise is to add a Faraday mask layer M to the transformer 11 of the transforming device 1. The Faraday mask layer M covers the secondary winding 112 and is electrically connected to the primary winding 111 so that the common mode current flowing from the primary winding 111 to the secondary winding 112 may be reduced. However, this method increases the capacitance of the secondary winding 112 with respect to the primary winding 111. Hence, it is quite difficult to suppress the common-mode noise.
Thus, it is an important subject of the invention to provide a transforming device of a power source and a transformer thereof capable of eliminating the common-mode noise so that the size and the manufacturing cost of the transforming device of a power source or the transformer may be reduced and the power density can be enhanced.